1. Field of the Invention
The present invention relates to a hydraulic elevator for ascending and descending a car by feeding a hydraulic fluid to a hydraulic cylinder or discharging it. More particularly, it relates to a hydraulic elevator which controls the speed of the car in the descending operation by regenerative braking of a motor for driving a hydraulic pump.
2. Description of the Prior Art
Heretofore, in the descending operation of the car of the conventional hydraulic elevator, the dead weight of the car and a plunger has been used and the speed has been controlled by controlling flow rate of a hydraulic fluid discharged from the hydraulic cylinder through a flow rate control valve.
In such descending operation, however, pressure energy is converted into heat energy at the throat of the flow rate control valve thereby causing severe elevation of the temperature of the hydraulic fluid to deteriorate the hydraulic fluid. Moreover, the flow of the hydraulic fluid passing through the flow rate control valve becomes high speed flow to cause cavitation and the noise and vibration caused by the cavitation are disadvantageously propagated to the car.
The following system has been proposed to overcome the above-mentioned disadvantages.
The motor is reversely rotated during the descending operation of the car to rotate reversely the hydraulic pump so as to discharge the hydraulic fluid from the hydraulic cylinder and when the descending speed of the car increases to increase the revolution speed of the hydraulic pump resulted by the discharge of the hydraulic fluid over the synchronous speed of the motor, the motor is actuated as the dynamo to apply regenerative braking to the hydraulic pump so as to control the speed. In such system, however, if the speed of the motor does not correspond to the opening operation of the check valve at the start of the descending operation of the car, the fluid is not fully fed in the suction side of the hydraulic pump to cause negative pressure and to disturb the opening operation of the check valve and to cause cavitation. Sometime, the hydraulic pump may be broken. In the decceleration, a similar problem occurs. Therefore, it has been required to correspond the starting and stopping operation of the motor to the operation of the valve. It has been required to control by fine control which is expensive.